使用CGD创建一个定时器

CGRunLoopTimerRef是基于时间的触发器，基本上说，就是NSTimer，他受到RunLoop的Mode影响，所以有点时候，我们说NSTimer可能会不准

但是CGD的定时器不受到RunLoop的影响

首先创建一个定时器

/** timer 本身就是一个对象，所以不要用*修饰,必须对他强引用，否则修释放*/
@property(strong,nonatomic)   dispatch_source_t timer;

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event{
    
    //创建一个定时器
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
    self.timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);
    //    dispatch_time_t start = dispatch_time(DISPATCH_TIME_NOW, 1.0 * NSEC_PER_SEC);
    
    NSLog(@"main  -%@",[NSThread currentThread]);
    dispatch_source_set_timer(self.timer, DISPATCH_TIME_NOW, 2 * NSEC_PER_SEC, 0 * NSEC_PER_SEC);
    dispatch_source_set_event_handler(self.timer, ^{
        NSLog(@"%@",[NSThread currentThread]);
    });
    dispatch_resume(self.timer);
}

数据打印:每隔两秒打印一次，非常准时

2017-04-20 10:48:42.129 WXAllTest[1274:26444] main  -{number = 1, name = main}
2017-04-20 10:48:42.130 WXAllTest[1274:26492] {number = 3, name = (null)}
2017-04-20 10:48:44.131 WXAllTest[1274:26492] {number = 3, name = (null)}
2017-04-20 10:48:46.130 WXAllTest[1274:26492] {number = 3, name = (null)}
2017-04-20 10:48:48.130 WXAllTest[1274:26492] {number = 3, name = (null)}
2017-04-20 10:48:50.130 WXAllTest[1274:26492] {number = 3, name = (null)}
2017-04-20 10:48:52.131 WXAllTest[1274:26492] {number = 3, name = (null)}
2017-04-20 10:48:54.130 WXAllTest[1274:26492] {number = 3, name = (null)}
2017-04-20 10:48:56.131 WXAllTest[1274:26492] {number = 3, name = (null)}
2017-04-20 10:48:58.130 WXAllTest[1274:26492] {number = 3, name = (null)}
2017-04-20 10:49:00.130 WXAllTest[1274:26492] {number = 3, name = (null)}

如果想3秒钟之后再去启动定时器，每隔2s执行某件事怎么办？

dispatch_source_set_timer(self.timer, DISPATCH_TIME_NOW, 2 * NSEC_PER_SEC, 0 * NSEC_PER_SEC);

第一个参数表示timer，第二个是开始时间，默认是当前时间DISPATCH_TIME_NOW,第三个参数是每隔几秒，单位是纳秒;
我们只要对第二个参数从新处理下就行，但是这个比较特殊，应该这样构建,否则无效

dispatch_time_t start = dispatch_time(DISPATCH_TIME_NOW, 1.0 * NSEC_PER_SEC);

停止一个定时器

//如何停止CGD定时器？
static int count = 0;
- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event{
    
    //创建一个定时器
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
    self.timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);
    
    
    NSLog(@"main  -%@",[NSThread currentThread]);
    //设置一下timer的开始时间和间隔
    dispatch_source_set_timer(self.timer, DISPATCH_TIME_NOW, 2 * NSEC_PER_SEC, 0 * NSEC_PER_SEC);
    //timer每次执行的事情
    dispatch_source_set_event_handler(self.timer, ^{
        NSLog(@"%@",[NSThread currentThread]);
        if (count == 4) {
            dispatch_cancel(self.timer);
            self.timer = nil;
        }
        count ++;
    });
    //启动timer，否则不能启动
    dispatch_resume(self.timer);
}

2017-04-20 11:00:14.659 WXAllTest[1501:38869] main  -{number = 1, name = main}
2017-04-20 11:00:14.660 WXAllTest[1501:39450] {number = 3, name = (null)}
2017-04-20 11:00:16.660 WXAllTest[1501:39450] {number = 3, name = (null)}
2017-04-20 11:00:18.660 WXAllTest[1501:39450] {number = 3, name = (null)}
2017-04-20 11:00:20.661 WXAllTest[1501:39450] {number = 3, name = (null)}
2017-04-20 11:00:22.660 WXAllTest[1501:39450] {number = 3, name = (null)}
最后只打印了这些，说明关闭了